F BICH 410 1st Edition Lecture 5 Outline of Last Lecture Standard Amino Acids Nonstandard Amino acids Outline of Current Lecture Should be able to identify parent Amino acid AA of non standard amino acid Not all amino acids are found in proteins some found with non protein roles o Ie neurotransmitters chemical messengers o Tyrosine parent of dopaminehistidine parent of histamine Ionizable groups in AA o All AA have at least 2 acid base groups ie alpha carboxyl alpha amino those with ionizable side chain have 3 o no net charge zwitterion o have to know pKA of all AA with ionizable side chain o alpha COOH pKa 2 2 alpha NH3 pKA 9 5 Nonpolar AA has no ionizable side chain so have just 2 pka Polar AA have ionizable side chain therefore most know the R group PKA o Arginine 12 5 o Cysteine 8 0 o Serine 13 o Glutamic acid 4 3 o Histidine 6 0 o Lysine 10 5 o Threonine 13 o Tyrosine 10 0 Ph PKA deprotonated polar soln less acidic low H deprotonated predominate PH pKA protonated polar soln acidic high H so protonated predominates Structure varies w PH o at low PH all ionizable groups are protonated cationic o at mid ph zwitterion dominates no net charge o at high ph ionizable groups are deprotonated anionic H removed Ionizable form 2 dissociable forms 3 structures Must be able to draw amino acids at specific PH These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o pH pKa protonated pH pKa deprotonated Titration Curves 3 pKA 3 dissociable protons and 4 ionic forms Ionization equilibria titration curve from lowest fully protonated to highest pH fully deprotonated Isoelectric point pH at which molecule carries no charge known as pI Calculating pI list all pKA in ascending order calc net charge for ph range that brackets each pka value identify 2 pkas that when titrated result in net charge from 1 0 1 use 2 pkas to calculate pI pI pki pkj pI of His 2 2 6 9 5 pH 2 2 pH 6 pH 9 5 pH 2 1 0 1 Pi 6 9 5 7 75 Unique Characteristics o Gly non chiral too flexible o Ile Thr 2 chiral centers o Pro cyclic not primary amino rigid not flexible o Cys 2 can oxidize and form disulfide bond does ox red rxn o Asn Gln amides forms of Asp and Glu
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